However, to play devil's advocate I'll ask; what's an appropriate bounty? If one were to offer a bounty of 0.5BTC for testing a service, how unbiased will the reviewer be? At what point does it start to look like something nefarious?
It used to happen in the mining section all the time, there were reviewers (probably still are) that were paid with crypto or hardware to review new miners. Some reviewers were paid some were not, iirc one high profile user was paid 3000
BTC to be a full on paid shill for a miner manufacture. I think it was open from the start he was paid, but it wouldn't have mattered as it was clear as glass where his loyalty was.
Everyone should always assume every review is paid for until you have established a personal comfort level with the reviewer (ie do they meet your personal criteria of providing unbiased reviews).